The present invention relates to magnetic random access memory, and more specifically, to reference cells for spin-torque based memory devices.
A spin torque magnetic random access memory (MRAM) device uses a spin-torque based memory element, for example, including a pinned layer, a tunnel barrier layer and a free layer in a magnetic tunnel junction (MTJ) stack. The magnetization of the pinned layer is fixed in a direction such that when current passes through the MTJ stack, the free layer becomes either parallel or anti-parallel to the pinned layer. Resistance of the MTJ stack depends on the relative orientation of the free layer and the pinned layer. When the free layer is parallel to the pinned layer, the MTJ stack is in a low resistance state (e.g., a “1” memory state) and when they are anti-parallel, the MTJ stack is in a high resistance state (e.g., a “0” memory state).
During the reading of data, a small current flows through the MTJ stack and its resistance is compared with a pre-written MTJ cell called a reference cell, to determine whether the MTJ stack being read is in a high or low resistance state. Reference cells are typically prewritten to be set to “0” memory state and “1” memory state. A problem associated with spin-torque memory devices is that the act of reading the reference cell may disturb the data. In order to read the resistance of the reference cell, current is passed through the reference cell and the voltage across them is measured. This current may accidently write the reference bit causing a “read” disturb.